The option to create a bootable USB does not exist when running boot camp assistant.
On Catalina 10.15.7 tempting to install Win10 via a downloaded iso file, but boot camp assistant doesn't offer a USB bootable creation tool. I.E., it instead requires insert/mount a Windows dvd installer disk.
Mac Pro
Check if this applies to your situation: Get started with Boot Camp Assistant on Mac - Apple Support
"Depending on the model of your Mac, you might need an external USB drive
to install Windows (you’re asked for one when you prepare your Mac for
Windows if it’s required). Newer Mac models—with OS X 10.11 or later—use
the internal drive to temporarily store the items you need to install
Windows and don’t require a USB drive. To find out whether you need an
external USB drive, see the “Learn more” section in the Apple Support
article Install Windows 10 on your Mac with Boot Camp Assistant."
The option to create a bootable USB does not exist when running boot camp assistant.
